(1) Field of the Invention
The present invention relates to an optical transmission device and an optical transmission system, and more particularly, to an optical transmission device for controlling the transmission of optical signals and to an optical transmission system for controlling the transmission of optical signals over a ring network.
(2) Description of the Related Art
Optical communication network technology is a core technology for constructing the infrastructure of information communication networks and is undergoing rapid development for the present-day information-oriented society in which highly advanced, wider-coverage services are demanded. Also, investigation is being made into network topologies that suit various fields of applications of optical communications.
FIG. 14 illustrates an optical communication ring system. Nodes 101 to 104 are connected in ring form by an optical fiber cable. To ensure the reliability in case of fault, the inter-node connection is configured as a redundant system including Work (active) lines and Protection (standby) lines.
In the case of ring topology like the illustrated one, basically there is no limit on the number of nodes that can be connected to the system. Ring topology is therefore suited for relatively large-scale networks and is often applied to optical LANs etc. for the trunks of local networks.
FIG. 15 schematically illustrates the internal configuration of a conventional node. The node 101 is connected to other nodes via the Work and Protection lines.
The node 101 comprises a signal processing section 101-1 and an electric switch section 101-2. The signal processing section 101-1 converts an electric signal supplied from the electric switch section 101-2 to an optical signal and transmits the resulting signal to other nodes. Also, the signal processing section converts an optical signal received from other nodes to an electric signal and supplies the resulting signal to the electric switch section 101-2.
The electric switch section 101-2 electrically switches signals to be supplied to and received from the signal processing section 101-1 and, in case of fault, carries out a switchover between the Work and Protection transmission lines.
While the above node uses an electric switch in its switch section, there has also been proposed a technique of using an optical switch in place of an electric switch. For example, Unexamined Japanese Patent Publication (KOKAI) No. 6-209284 discloses using a plurality of 2×2 (2-input, 2-output) optical switches to perform switchover between transmission lines.
In the aforementioned conventional node provided with an electric switch section, however, in order for the electric switch section to process high-speed data, it is necessary that the node be provided with a DMUX section for converting high-speed data to low-speed data so that the electric switch section can process the data, as well as an MUX section for converting low-speed data output from the electric switch section to high-speed data. A problem also arises in that increase in transmission speed entails enlargement of the scale of the circuitry of these converting sections.
Further, when the high-speed data/low-speed data conversion is performed, an increased number of signals are input to and output from the electric switch section. In such cases, interface signals on the backboard or in the unit also increase, giving rise to a problem that the electric characteristics deteriorate due to crosstalk or the like.
On the other hand, the aforementioned conventional technique using 2×2 optical switches provides only the function of performing switchover between the active and standby lines that carry the ring-side high-speed data. Thus, where a terminal office is connected to the node, no line switching is performed with respect to the terminal office, so that the system lacks expansibility.
Also, since neither fault information extraction process nor device status information setting function is available, a problem arises in that a faulty spot cannot be located.